It is estimated that there are almost 16 million bone fractures in the U.S. each year. While many of these fractures occur following trauma, an increasing number of fractures occur with only minor trauma as the incidence of osteoporosis increases and the population ages. In particular, aging is associated with poorer healing. Thus, fracture and healing have a substantial impact on overall health, mobility, quality of life, and the health care system. Although most fractures heal normally or with minimal surgical intervention, approximately 5-15% of fractures display delayed healing or nonunion. When nonunion occurs, it is associated with significant morbidity, prolonged pain, additional surgical procedures, reductions in the quality of life and loss of productivity. Cell-based therapies for bone healing, particularly the use of mesenchymal stem or stromal cells (MSCs) that can differentiate into osteoblasts, adipocytes and chondrocytes, have been the focus of preclinical and clinical investigation and represent an attractive adjunct to current standard methods. Studies have shown what appears to be an inverse relationship between the differentiation of MSCs along either an adipocyte or osteoblast lineage such that agents that stimulate adipocyte differentiation simultaneously inhibit osteoblast differentiation and, conversely, agents that stimulate osteoblast differentiation simultaneously inhibit adipocyte differentiation. The overall goal of this proposal is to take advantage of the known interplay between adipose cells and osteoblasts by manipulating specific genes involved in adipose metabolism to favor the differentiation of MSCs into bone and, thus, accelerate bone healing following injury. Specific aim #1 will test the hypothesis that global knockdown of hormone sensitive lipase (HSL) will accelerate bone healing in mice using a bone injury model that recapitulates fracture healing. This will be accomplished by comparing the ability of wild-type control mice and HSL null mice to heal a tibial injury model that creates a non-critical bone defect. Specific aim #2 will examine the impact of knockdown of HSL in bone marrow MSCs on bone healing in mice with a critical bone injury model. This will be accomplished by comparing the ability of bone grafts derived from bone marrow isolated from either wild-type control mice or HSL null mice to accelerate bone healing when implanted into mice with a calvarial bone injury model that will not heal without implantation. Successful completion of this SPiRE will lay the groundwork for a full RR&D Merit proposal that will explore the biologic basis for this approach in more detail and further evaluate the potential for its clinical translation.